FIG. 1 illustrates is a schematic diagram of a conventional uncorrected switching regulator power supply 10 which converts unregulated AC high voltage from an AC power line to regulated DC low voltage for powering circuitry in electronic equipment such as computer terminals, radar transmitters, machine tools, motors, and controllers.
The uncorrected switching regulator power supply 10 shown in FIG. 1 includes a full wave rectifier component 15 adapted to receive the 3 phase 120/240/480V, 50/60 HZ power line voltage. The rectifier input is connected to a filter 17, capacitor filter, in turn, this is coupled to a DC to DC inverter circuit 20. The inverter 20 supplies a transformer 25 having a primary winding N1 and a secondary winding N2. The electrical output waveform 50, referred to as Eo, at the secondary of the transformer, is shown. This waveform 50 is applied to a low pass filter 60 constituting the output circuit of power supply 10, comprising inductance Lo and output capacitance Co.
The AC line voltage is full wave rectified by rectifier 15, and filtered by 17, resulting in a high voltage unregulated DC bus voltage at the output of the rectifier. This filtered voltage is applied to the inverter circuit 20. The latter is a DC to DC inverter which typically operates from 20 to 500 KHZ, and a control circuit provides pulse width modulation to the switching devices for control of the output 75 DC level. The DC output component 75 of the input waveform from the AC power line is the output voltage of the low pass filter circuit 60 (and, thus, of power supply 10), it is the ratio of the width of each pulse (ton) to a full cycle (T) of the train multiplied by the pulse amplitude, or mathematically as shown in FIG. 1: Output voltage=(ton/T)(N2/N1)*EDC where ton is pulse width; T is pulse cycle time; N 1 and N 2 are the number of windings on the primary and secondary, respectively, of the transformer; and EDC is the DC level of the rectified AC input voltage to the power supply across capacitor 17.
As shown in the plot of FIG. 2, the amplitude of the input current waveform 80 consists of a periodic series of quasi sinusoidal current pulses, each pulse corresponds to the conduction interval of the input voltage 12, full wave rectifier 15, capacitor 17 (as shown in FIG. 1). That is, the input current will flow whenever the input voltage per phase is greater than the capacitor voltage 17. As such the input current will contain intervals of zero 85 current and spike intervals of current 87. The resultant input current per phase is shown in FIG. 2. As seen, this is rich in harmonic current levels and the total harmonic content can exceed 50%. Each phase input current will be the same as FIG. 2, shifted by 120 degrees in time.
This output voltage from power supply 10 is a suitable low voltage supply for any of a number of electronic equipment applications, such as computer systems, medical instrumentation, telephone switching systems, machine control systems, or other apparatus employing semiconductor devices, motors or integrated circuitry or that requires supply voltages.
The output voltage of the supply 10 is employed as the supply voltage for any device operating on DC power. The power supply efficiency is the ratio of power out to power in, and can be high—for example, greater than 80% for 150 volt outputs and greater than 75% for 48 volt outputs. The power factor, which is a measure of how well such a power supply utilizes the AC line voltage, however, is typically relatively low. A power factor of 0.70 is not unusual for supplies above 10,000 watts.
Low power factor is attributable to the fact that the input current drawn by the rectifier and filter capacitor of the power supply is not sinusoidal and is not in phase with the input voltage. For example, as shown in the plot of FIG. 2, one phase current 80 of the three phase line current drawn by the supply, from the source (e.g., a 7.5 KW power supply load, 480V AC/60 HZ operation), is drawn only in periodic pulses 87 to recharge the input capacitor. A power factor improvement can be realized by increasing the conduction angle φ, but this capability is limited by the ripple current rating of the input filter capacitor. For a typical conventional power supply with a conduction angle “φ” of ¼ of T/2 seconds, the demand is four times the RMS value of the input current.
As shown in the plot 80 of FIG. 2, the large peak current loading produces stress on the facility source, and may result in loss of peak AC voltage because of reactive and resistive regulation losses. It is not unusual to measure harmonic current values of greater than 50%. This far exceeds good design practice for AC loading of a source and generators in particular, for long life.
In a further example, a three phase power supply having a power factor of 0.75 draws 25% more input current than a comparable power supply having a unity power factor. For example, a conventional 7500 watt, 425 volt power supply operating with a 0.75 power factor off a 480 volt three-phase AC input line will draw 12 amps. The harmonic current content will exceed 50%. This harmonic current per phase is further detrimental if a neutral is present since harmonic currents will add on the return neutral connection.
The susceptibility of other loads to deterioration of performance is reduced in the presence of a power supply operating with the higher power factor, at least partly because harmonic current is substantially reduced or virtually eliminated.
It would be highly desirable to provide an improved regulated DC power supply with high efficiency of power conversion, reduction of line harmonic current and providing near unity power factor.